older Stax headphones

I have a Stax SRD-6 amp and SR-5 headphones. These are from the early 1980's. They take a six pin "low bias" cable. I haven't been able to locate a headphone extension cable that has the six pin configuration, all the current Stax cables are five pin. Any ideas or suggestions? It would make listening so much easier if I could fine a two meter extension.